The present invention relates to photomultiplier tubes and, more particularly, to a photomultiplier tube in which uniform transverse spacing is provided in the electron multiplier cage assembly to prevent distortion of the elements comprising the cage assembly.
In the manufacturing of a high volume photomultiplier tube, conflicting objectives are frequently encountered. The tube must be designed to provide accurate and reproducible measurement of the phenomenon being observed, and yet the tube must be inexpensive to produce. The former objectives demand that the position of each element of the tube be fixed accurately with respect to the other tube elements; however, this must be done without the use of a large number of expensive precision parts. In order to achieve the latter objective, the tube must be easy to assemble in order to eliminate the need for difficult, time consuming assembly techniques.
In one type of photomultiplier tube, the electrodes of an electron multiplier are supported on metal brackets having arms which protrude transversely through large apertures formed in parallel insulating spacers disposed on either end of the electrodes. Electrical leads are welded to the metal brackets and to the free ends of some of the auxiliary electrodes. Others of the auxiliary electrodes have mechanical stops welded thereto. The brackets maintain the integrity of the electron multiplier. One such structure of this type having an inline electron multiplier is shown in U.S. Pat. No. 2,200,722, issued to Pierce et al. on May 14, 1940. The Pierce et al. structure is complex, labor intensive and, thereore, expensive to manufacture.
U.S. Pat. No. 4,125,793, issued to Timan on Nov. 14, 1978, discloses a photomultiplier tube having a box and grid dynode structure with a floating anode that is not supported by the insulating side supports which rigidly hold the dynodes in fixed spatial relation. The purpose of the floating anode is to isolate the anode from the side supports which are disclosed to carry ohmic leakage or leakage currents. There is no disclosure concerning the manner in which the dynodes are affixed to the side supports.
It has been found that in an electron multiplier structure, such as that disclosed in the Timan patent, in which a dynode mesh extends across one or more of the dynodes, transverse pressure exerted by the insulating side supports frequently deforms the dynode mesh to the point where the mesh alters the electrostatic field of the dynode causing a decrease in tube performance. Severe distortion of the mesh may even result in the mesh contacting an adjacent electrode causing an electrical short.
It is therefore desirable to provide a means for establishing a uniform minimum spacing between the insulating supports which support the dynodes and the anode of the electron multiplier so that there will be no deformation of the elements comprising the electron multiplier.